1. Field of the Invention
This invention relates to electronic methodology and apparatus for synchronizing the timing of a sequence of electrical output pulses with a sequence of electrical input pulses in accordance with a desired correlation therebetween. More particularly, the invention is concerned with making it possible and practicable to accomplish such synchronization in accordance with virtually any linear, non-linear or empirical correlation function, which it may be appropriate to select for any given purpose.
Still more specifically, the preferred embodiment of the invention chosen as a basis for illustrating and explaining the application of its principles to an area of technology where it is especially needed at this time is concerned with providing a significantly improved technique and system for more accurately reading binary data from magnetic disk devices and the like.
In the last-mentioned context, I have found a specific correlation function which I prefer to employ for that particular application, and it is noted hereinafter. However, it should be understood at the outset that, neither in the more restricted context of application to the reading of binary data from magnetic storage media, nor in its more general context, is the invention primarily directed or essentially limited to any particular correlation function. Rather, the invention is concerned with providing a method and apparatus by which substantially any selected correlation function of whatever nature can be accurately, reliably and economically applied in the synchronization of sequences of electrical pulses in any manner selected as useful for any particular purpose.
2. DESCRIPTION OF THE PRIOR ART
In general, the classical approach to synchronizing the frequency of one alternating current signal with another has involved the employment of analog type circuitry arrangements of various kinds, often referred to as "phase locked loops," in which differences in frequency or phase of the output signal from a reference input signal are sensed and "fed back" in some suitable manner to the source of the output signal to alter its frequency or phase so as to bring it in to conformity or synchronization with the reference input signal, the kind of correlation most typically sought in such instances being simply full correspondence between output and reference inputs. The same sort of analog techniques have been attempted with pulse type signals in certain applications, but with less than fully satisfactory results, particularly when the desired output signal is to be correlated with the input signal in some manner other than merely direct correspondence therebetween.
Various digital techniques (and hybrid digital and analog arrangements) have also been proposed for use in synchronizing pulse sequence type signals; for example, see: U.S. Pat. Nos. 3,364,439 of Cohen et al, 3,931,585 of Barker et al, 3,936,762 of Cox et al, and 3,983,497 of Malek. However, such prior digital systems have essentially been based upon integer divisions of frequency, perhaps with alteration of the divisor, and have each been subject to one or more of a number of disadvantages or limitations, such as requiring complex and expensive, hard-wired logic circuits, inherent inability to realize any but a restricted class of correlation functions, inherent inability (at least without substantial modification) to function with various types of commonly occurring and commercially related input signals (such as derivable from the different species of encoding schemes used to record binary data on magnetic disks), inherent inability or impracticality for keying synchronization to an input having a varying frequency or irregularly occurring pulses (such as the pulse sequence derivable from the sensing of magnetic state transition boundaries during reading of binary data stored on a disk device), etc.
I am not aware of anyone having previously taught or employed the straigtforward, reliable, accurate, economical and, perhaps retrospectively, simple approach to providing a generalized solution to the problem of synchronizing pulse type signals in accordance with virtually any selected linear or non-linear correlation function, as achieved by my invention and hereinafter described.